One prior art approach for joining a ceramic funnel wall of a cathode-ray tube envelope to a ceramic ring wall of such envelope employs a pair of couplers for this purpose. Each of these couplers is formed of two annular pieces. The first piece is of Ni-Cr-Fe alloy and is of L-shaped cross section with a flat base ring portion and a rectangular-tubular portion which projects from one edge of the base portion. One such alloy is sold under the brand name Sealmet. The second piece is a nickel-iron alloy flat rectangular ring which is brazed to the projecting edge of the tubular portion. To join the ceramic funnel wall to the ceramic ring wall, a multiple-step process is used. First, the base ring portion of one of the couplers is fritted to an edge of the ceramic funnel wall and the base ring portion of the other coupler is fritted to an edge of the ceramic ring wall. The faces of the base ring portions are then positioned to abut one another. Thermal clamps are then temporarily fastened in place in contact with the nickel-steel rings. These clamps provide a heat sink and hold the couplers together. The base ring portions are then welded together at high temperature by tungsten inert gas (TIG) or plasma welding. Thereafter, the thermal clamps are removed.
In the above approach, the use of a thermal clamp is relatively time consuming, but is required to prevent shattering of the ceramic components and failure of the frit joints. During such TIG or plasma welding, the temperature of the couplers rises significantly. Moreover, the frit joint will fail if a large temperature differential exists between the metal couplers and ceramic.
In addition, the coefficients of thermal expansion of the ceramic, which may comprise forsterite, also sometimes called fosterite, and the frit are extremely close. However, the coefficient of thermal expansion of the Ni-Cr-Fe alloy varies significantly from these other coefficients of thermal expansion. This variation occurs over the range of working temperatures to which the frit joint is subjected during welding, processing and also during use of the cathode-ray tube envelope. Consequently, the coupler, frit and ceramic expands and contracts differing amounts and at differing rates. This can lead to cracking of the ceramic and also to failure of the frit joint. This problem is further compounded by the fact that the coefficient of thermal expansion of Ni-Cr-Fe alloy varies depending upon the range of temperatures reached by the alloy prior to use in manufacturing a coupler. Thus, depending upon its thermal history, quantities of chemically identical Ni-Cr-Fe alloy can have different coefficients of thermal expansion.
Furthermore, this prior art approach results in an expanded cathode-ray tube envelope. That is, to accommodate the necessary thermal clamp, the tubular portions of the couplers are typically about one inch high. Therefore, following their connection, the ceramic components are approximately two inches apart, and in the case of a cathode-ray tube, are nearly equal in cross section to the front plate of the tube. Consequently, enlarged cabinetry is required to accommodate oscilloscopes and other equipment which uses cathode-ray tubes with such envelopes.
Thus, the above approach requires time consuming steps to interconnect ceramic components, results in ceramic to ceramic couplings of less than optimum compactness, and provides ceramic to ceramic couplings which suffer somewhat from a lack of reliability.
U.S. Pat. No. 2,912,340 of Pincus discloses a forsterite ceramic material used in vacuum tube envelopes. FIG. 2 of this patent shows metallic discs of titanium, zirconium, or alloys thereof which are sealed to ceramic members 33 and 34. This patent mentions, at column 7, line 64, that these elements are sealed by any known satisfactory soldering or brazing technique. Also, column 7, line 76, through column 8, line 8, discusses the necessity that ceramic elements 33 and 34 have thermal expansion and contraction characteristics which closely approach those of titanium so as to avoid rupturing the ceramic or the seal between the metallic and ceramic elements.
In the Pincus patent, the soldering or brazing techniques are understood to be relatively high temperature techniques (700 degrees Centigrade and higher). In addition, titanium is brazed in a vacuum, which would require a relatively expensive vacuum oven. Furthermore, due to the high temperatures involved in such brazing, frit joints would be destroyed unless the brazing was accomplished in a separate step before fritting. This would add to the time and cost of manufacturing these devices. As another drawback, the high temperatures employed by these techniques would melt glass. This makes such techniques totally inappropriate for interconnecting glass components.
It should also be noted that laser welding of titanium to itself is known in the prior art. In addition, a fillet welding technique is known in which one edge of a first component is positioned to overhang an edge of a second component and then these edges are welded. However, the inventors of the present invention do not know of any use of laser welding in applications in which titanium is also previously fritted to ceramic components.
Therefore, a need exists for a method and apparatus for interconnecting and hermetically sealing ceramic components, which overcomes these and other disadvantages of the prior art.